Today's consumers are demanding increased functionality of wireless transceivers whilst simultaneously requiring a reduction in transceiver size and cost. To cope with these conflicting demands, most of the current generation of wireless transceivers use one of two receiver architectures, namely the direct conversion (or zero intermediate frequency [ZIF]) architecture and the very low intermediate frequency (VLIF) architecture. However, both of these architectures suffer from problems associated with DC offset signals and I/O imbalance.
Typically the incoming signal to a wireless transceiver will consist of a desired signal and a host of other signals which will be classed as undesired signals or interference. The desired signal will occupy a range of frequencies which is typically known as either the desired signal band or channel. In this context the two terms are interchangeable. Channel is more likely to be used when referring to the frequency range of the desired signal when it is transmitted, as this is strictly specified by standard. In contrast, at the receiver, desired signal band is more likely to be used, as the primary metric here is bit error rate. Furthermore, the receiver can define its own desired signal band, as long as the receiver meets performance requirements.
U.S. Pat. No. 6,577,855 describes a method of inverting the sign bit of a VLIF in response to receiver signal characteristics such as bit error rate (BER). However, the scheme described in U.S. Pat. No. 6,577,855 merely swaps image bands to reduce blocker interference and does not balance the contribution of all sources of impairment. In other words, the prior art adjusts the sign of the VLIF in response to a performance metric but does not consider the effect of other sources of impairment that are impacted by the choice of the VLIF. In particular, the prior art neither considers the impact of a DC offset generated by the receiver itself, nor the effect of the circuitry required to remove the offset